Basic usage of kotlin

1、 summary

“ If you are for Android Develop and find an alternative programming language , Then we should try Kotlin. It's easy to be in Android Substitute... In the project Java Or the same Java Use it together .”–Hathibelagal

Kotlin v1.0 On 2016 year 2 month 15 Promulgated by the . This is considered the first official stable version , also JetBrains Ready for long-term backward compatibility starting with this release .
stay Google I/O 2017 in ,Google Announce on Android Up for Kotlin Provide the best support .

Time passed quickly ,Kotlin Language has been released 3 Years. , As Android For developers ,kotlin Language will become a new language that we have to master , Especially kotlin Language introduction process （Coroutine） After this feature , More obvious advantages .

Kotlin+Mvvm Such a composite architecture has gradually become the mainstream , Bloggers are using this architecture for project development , The amount of code is greatly reduced , The frame structure is clear , I recommend you to learn and master , Later, I will write the architecture used in the project independently , Welcome to Follow this blog follow-up .

2、 How to create a class

for instance ：
ResLogin.kt

class ResLogin {
    
    var code = -1
    var msg = ""
    var token = ""

    lateinit var retObj: RetObjBean
    inner class RetObjBean {
    
        var id = 0
        var name = ""
    }
}

Corresponding Json character string ：

{
    
    "retCode": 0,
    "retMsg" : "success",
    "retObj ": {
    
        "id":378,
        "name":"admin"
    },
    "token" : "3b6abdb79b29d6ce33b71c9da5d30de6"
}

After the attribute declaration, it will be automatically generated get,set Method , We use it directly through （ Class . Property name ） Perform assignment and value taking operations .

for example ：

var login = ResLogin()
login.code = 0 // assignment   amount to set Method  
print(login.retMsg) // Value   amount to get Method  

And java There are differences in classes 4 It's about ：

（1）、var：

Declare keywords for attribute variables , The data type of the attribute can be automatically inferred from the assignment

（2）、lateinit ：

This keyword yes late init A combination of two words , That is, delay initialization , You don't need to copy it as null, Instead, wait until you need it to be used .

Be careful ：lateinit Modification cannot modify the original data type （byte,char,short ,int,long,float,double）, But it can be decorated String, And custom class Class etc. .

When we don't add this keyword, we will report the following error .

lateinit Refer to

（3）、 inner class：

And java Of static class Declare that the usage of static inner classes is the same , Just different keywords , because kotlin No more static Keyword. .

（4）、lateinit var retObj: RetObjBean：

Declare a as RetObjBean Type of retObj attribute . This is a strange way of writing , The type of property is ： Post written , And java Completely different , So you need to get used to .

3、 Subclass inherits parent

for instance ：
ProjectApplication .kt

class ProjectApplication : Application() {
    
    override fun onCreate() {
    
        super.onCreate()
        LitePal.initialize(this)
        context = this
    }

    companion object {
    
        lateinit var context: Context
    }

}

And java There are differences in the way inheritance is written 2 It's about ：

(1)、 :Application()

Use ：+ Parent class name （） This way to inherit the parent class , Different from our original writing ,： It stands for inheritance ,Application Represents the name of the parent class , （） Represents the default constructor of the parent class .

(2)、override fun onCreate()

The method of overriding the parent class also changes ,override Rewrite keywords for ,fun Represents the keyword that declares a method and class The key is the same , Similar action ,onCreate Method name ,() Is a formal parameter .

Let's focus on this code ：

companion object {
    
        lateinit var context: Context
    }

companion object ： translate become The meaning of the accompanying object , Belongs to the class itself , therefore companion object It can't be defined in the global without class .
It is similar to Java Inside static Variable , Direct class name dotting call .
Such as ：

4、 Declare a function with a return value

for instance ：

/** *  Date formatting  * * @param date  Date to format  * @param pattern  Time format , Such as ：yyyy-MM-dd HH:mm:ss * @return  Return the formatted time string  */
fun format(date: Date, pattern: String): String {
    
    val sdf = SimpleDateFormat(pattern)
    return sdf.format(date)
}

You can see that the formal parameter declaration is the same as the attribute declaration ： Parameter type after colon .
Notice the return value information of the function , Directly follow the parentheses of the function with （： return type ） The way .
Calls and java It's the same .

5、for How to write a cycle

and java There is a little difference in the way of writing , Turned into in and … keyword .
for instance ：

for (index in 0..3) {
    
	print(index.toString())
}

Output results ：

0 
1 
2 
3

6、 summary

The content of this article is relatively simple ,kotlin A shortcut to language learning , Just use it 、 Use it 、 Or use it . Only skilled use , Corresponding to the back MVVM The combination of architecture will be of great benefit .

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